During a bonding operation, the bonding wire is pressed with a predetermined pressing force against a chip metallization, in order to achieve an intimate connection between the bonding wire and the chip metallization. However, with increasing diameter of the bonding wire, the pressing force required for this increases and, if the bonding is performed by means of ultrasonic bonding, the ultrasound output required increases. If, however, the power semiconductor chip is a device with an active cell region with a multiplicity of cells connected electrically in parallel, for example a MOSFET (metal-oxide-semiconductor field effect transistor) or an IGBT (insulated gate bipolar transistor), there is the risk with high pressing force and ultrasound output that the cell structure, and with it the power semiconductor chip, will be destroyed if the bonding of very thick bonding wires with a diameter of, for example, more than 100 μm or more than 300 μm is performed above the active cell region.
Therefore, very soft wire materials are typically used for bonding above active cell regions with thick bonding wires, typically of high-purity aluminum with a degree of purity of 99.99% or 99.999%, the microstructure of which is distinguished by large individual grains and a low hardness, for which reason a comparatively low pressing force and ultrasound output are required when bonding. As a result, the active cell region located under the chip metallization is subjected to loading of a lower level during bonding than in the case of harder wire materials.
Since, however, the coefficient of thermal expansion of aluminum differs very greatly from the coefficient of thermal expansion of the semiconductor material, for example silicon or silicon carbide, the bonding connection is exposed to considerable alternating temperature loading when it undergoes frequent temperature changes over large ranges, as occur in particular during switching operation under severe load changes, for example in the case of applications in the traction area. On account of such instances of alternating temperature loading, the contact area between the bonding wire and the chip metallization of the power semiconductor chip is reduced over time, until the bonding wire finally becomes detached from the chip metallization (“lift off”). The lift off problem is becoming more acute as development progresses, leading to ever higher permissible barrier-layer temperatures of the power semiconductor chips, and consequently to higher alternating temperature loading.
To reduce the risk of bonding wire detachment, it has in the past even be acceptable to take costly measures. Such a measure known from DE 10 2005 028 951 A1 is that of sealing the bonding location with polyimide, which however requires an additional cost-intensive process step during the bonding.
Another such measure comprises a bonding process with a large shearing area, as described in the publication of the paper by Siepe and Bayerer: “Time and spatial resolved detection of power device failures during wire bonding” at the CIPS 2006, 4th International Conference on Integrated Power Electronics Systems, Jun. 7 to 9, 2006, Naples/Italy, VDE-Verlag, Berlin, Frankfurt, ISBN 978-3-8007-2972-2. However, this method requires that the chip metallization undergoes a high level of loading and, when bonding above an active cell region, leads to a significant increase in the reject rate. Furthermore, the aluminum bonding wire used recrystallizes due to the high temperature and the alternating temperature loading, and as a result alters its mechanical properties. For example, a thermal treatment for four and a half hours at 190° C. on an aluminum bonding wire with a diameter of 350 μm leads to a reduction in the Martens hardness of about 20%. Furthermore, the breaking load of a pure aluminum bonding wire falls very sharply.